Check private key when decrypting from CLI

[mpnd]

I notice an interesting behaviour when testing CLI. When trying to decrypt an encrypted number using an "incorrect" private key (eg, one that has different keysize to a public key used during encryption) one of the following happens:

• IllegalArgumentException is thrown because the ciphertext of the encrypted number is greater than the public key's modulus.
• DecodeException is thrown because the decrypted value cannot be decoded as a double.
• successful decryption process, but the value is incorrect.

It would be good if CLI checks whether the correct private key is used (especially when the encrypted number is successfully decrypted).

[unzvfu]

Cf. #66